Electric detector and frequency changing system



F. E. TERMAN 1,812,066 ELECTRIC DETECTOR AND FREQUENCY-CHANGI ING SYSTEM June 30, 1931.

Filed March '7, 1927 gnoantoz (9 M 7 6mm Patented June 30,1931 1 UNITED STATES PATENT OFFICE i FREDERICK nMiaoNs TEn-r/mn, or srnuronn unlvnns rir, o LrroRNLA; nnormc nnrnczron AND rnEouENctrcnANemG SYSTEM Application filed March 7, 1927. Serial Nd. 173,419

I ly necessar The present invention relates'to an apparatus for detecting an electrical current modulated upon a current of higher fre-.

quencies, and to a'novel form .of frequency 5 changing system employingsaid detector.

Ordinarily, inthe receptionof wireless or radio waves, detection of the waves is'accomplished by eitherthe grid leak and grid condenser system or by theuse of a negative bias upon the grid of the audion tube, a s0-called C battery method. Where the detectionv is accomplished using'the grid leak-and grid condenser method, the efficiency is'several times higher than that using the C battery system. However, detection using the grid leak and grid condenser is limited to where the frequency of the detected current is not more than about five thousand cycles per second. This is for the reason that as the fre- 'quency ofthe detected current increases, the

grid condenser acts as a short circuit around the grid leak at the frequency being handled, a

f grid leak-condenser method of detection.

with the result that no detection occurs be tween the grid and filament of the tube, and if any detection is obtained in the tube at these frequencies, it takes place between the filament and plate of the tube, which is the mode of operation of the ordinaryC battery detection. The ordinary grid leak-condenser method of detection is operated for detection of ordinary audio frequencies, but when the frequency of the detected'current is high,

such for example as fiftythousandcycles per second, no detection'with the grid leak and grid condenser system is obtained between the grid and filament of an audion tube, but j the ineificient C battery detection is, utilized,

i; e.,the detection occurring between the plate and filament. For this reason, the sorcalled 40 first detector of a superheterodyne radio re- 'ceivingcircuit operates with very 10W, ef-

ficiency, the first detectorcausinga'loss in tlie signal strength so greatthat one ortwo stages of radio frequency amplification are general; 'y to restore the signal to its orig-' inal strength. j j i .It is the object of the presentfinvention to provide a system of detection'which will operate efficiently at any frequency of the detected current, producing eificient detection or rious objects and advantages thereof, will in connection with the tem of detection of the present invention.

in which the change of frequencyis accompli'shedwithout the usual high loss in signal strength. V v I I The present invention, together with va.

best be understood froma description of the, new system of detection and frequency changing system embodying said system'of detecf 'tion, and a comparison thereof Withthe normal grid condenser and grid leak system of detection. For this-purpose, there is herein after described a preferred form or forms of apparatuses and methods embodying the present invention, thef'same'being described ,7

7 accompanying draw:

ings, in which,

Figure 1 is a diagram illustrative of the Figure 2 is a diagram of a method or sys- Figure 3shows a modified system of detection. o v Figure 4shows another modified systg f detection; U I i Figure 5 shows another modified system of detection. I

Figure 6'" discloses asystem of detection with a neutralizing means for preventing OS- cillation of the circuiti' F gure'? shows a modification of Fig. 6,;

Figure 8 shows a similar modification, and

' Figure 9, illustrates a complete frequency changing system embodied in the present invention. j In orderto understand the operation of the present method of detectionpreferenceis first made to Fig. 'lywhich discloses the norvma-l. detection ofthe grid leakand grid con- "denser system. In such figure, 1' indicates: a'

tri-electrode tube/having a grid 2, plate 3 and l filament 4.. Connectedin the circuit leading to the grid Qis a'high resistance 5'shunted by a capacity 6.. In this type of'detector,*the grid condenser used should have atlea'st five or ten times the capacityo'f the grid-to fi'laf ment capacity of the tube 1. At the same time, this condenser should not be too large. The resistance 5 is normally a high resistance, ranging from one-half to ten megohms.

In operation of such a system of detection, normally a small grid current flows through the grid leak 5 due to the normal grid potential that exists in the detector, the current passing to the filament When a radio fre quency voltage'is applied to the detector, the grid condenser is practically a short circuit to this voltage so that the voltage is passed on and applied to the grid of the detecting tube. The result of this radio frequency voltage thus applied to the grid of the detecting tube is to cause an additional direct current to flow in the grid circuit. When the radio frequency voltage that is applied to the detector 1 varies in amplitude or is modulated, as is commonly called, for example, with an audio frequency current, the rectified current varies in amplitude in accordance with the fluctuations and amplitude of the radio frequency.

When two radio frequency voltages of different frequency are superimposed and applied to the detector tube 1, the rectified current that is produced will be an alternating current of the difference in frequency of the two radio frequency voltages. Also in addition to this difference frequency, there will be a sum frequency current. The rectified current that is produced by the application of radio frequency voltages to the detector must pass through the combination of grid leak and grid condensers 5 and 6, and in passing through this impedance will produce a voltage drop. This voltage drop changes the grid potential from the normal by an amount equal to the drop in the grid condenser-grid leak combination, and due to the amplifying action of the grid in the vacuum tube, the current from the plate 3 of the tube will vary in proportion to the change of the potential on the grid 2. In order, however, for any substantial effect to be produced, the impedance of the grid condenser and grid leak combination must be very high, usually in the order of a megohm or more. In this case, the rectified current will produce a considerable voltage drop in the grid circuit impedance made up of resistance 5 and condenser 6, and will accordingly produce a rather large change of grid potential, thereby giving a correspondingly efi'ective detective action.

In case where the frequency of the rectified current is moderately high, such as about five thousand cycles per second, it is found that the minimum size of grid condenser that can be employed and still by-pass the applied radio frequency voltage is so large as to practically short circuit the grid leak relative to the converted current, and reduce the impedance of the grid leak 5 and grid condenser 6 to such a low value as to produce little or no voltage drop in this impedance; thereby the current from the plate 8 is not substantially affected and a poor detection action is obtained. t frequency such as fifty thousand cycles per second, there is practically no detector action going on in the grid circuit, and that detection which is normally obtained takes place in the circuit of the plate 3, and is due to the fact that the plate current is not proportionate to the volage applied to the grid 2. The rectifying action that takes place from "this cause, however, is from six to fifteen times as inetiicient as that which can be obtained by efiicient rectification in the grid circuit.

The system of the present invention provides an efficient means for detection in that it embodies the following three essentials necessary in order to obtain good detection or rectification in the grid circuit of a detector. These conditions are:

(1) That the impedance that is placed in series with the grid. of the detecting tube is low with respect to the frequency of the voltages that are applied to the detector to be rectified.

(2) That the impedance that is placed in the grid circuit is very high or offers a great opposition to-the passage of the rectified current.

(3) That there is provided a passage whereby a steady normal grid current is present when no alternating voltage is applied to the grid.

As before explained, the ordinary grid leak-condenser method of detection fails to satisfy these requirements when the rectified current is not within the audio frequency range, since the grid condenser begins to bypass the rectified currents of such frequencies.

Now referring to Fig. 2 of the drawings, 7 indicates an audion tube with a grid 8, plate 9 and filament 10. In the circuit of the grid 8, there is placed an impedance 11, shunted by a condenser 12. The condenser 12 must ordinarily have a capacity as small as possible and still be capable of by-passing the frequency of the oscillatory energy applied to the grid 8. This means that the capaci y should be at least three to five times as great as the capacity between the grid 8 and filament 10, which, in the case of the ordinary form of such tubes, needs a capacity of, for example, one hundred or two hundred micro-microfarads. The inductance 11 should possess such a value relative to the condenser 12 as to form a resonant circuit at the frequency of the rectified current to be formed in the tube 8. For example, in the case of a frequency changing circuit employing the so-called superheterodyne principle, the first detector must handle a rectified current of about fifty thousand cycles, and accordingly, with the present invention there will be placed an inductance and condenser combination in the grid circuit of the first detector which would betuned to approximately fifty thousand cycles. In such a case the impedance offered by the inductance-condenser combination, 11 and 12, to the rectified current will be extremely highandcanbemade'in' the order of a megohm or greater;

thereby, thesystem meets the second requirement of good detection l/Vhen applying the circuit shown in Fig.

rectified current 1s approximately'the same for theentire bandof frequencies ofthe-rect-ified current. 5 e v v I It is understood, of course, that the'induc tance capacity system of employing impedance to the rectified currentin the. grid, as

illustrated inFig'; 2, is notnecessarily limited to casesrwhere the rectified current is high- 1 i in frequency, but may be applied where the rectified current is of 'any'frequen'cy. 'For example, if the rectified frequency was low, such as onethousand cycles, tl1e inductance 11 and condenser 12 shouldfbe'tuned to such one thousand cycles. The system of detection or revivification illustrated in Fig. 2 meets the first requirementof gooddetection, since the capacity 12offers substantially a short circuit to the .ordinary 'range of radio frequencies or the ordinary frequency of the oscillator of a superheterodyneapplied to the detector; Moreover, the inductance 11 provides a pathfor a steady discharge of current from the grid'when no radio fretube 8;

It is understood that inpractical construction a coil to produce the inductance 11 may quency voltage is applied to the detector 1 possess sufficient internal capacity asto fulfill the requirement of the condenser 12- so that the coil 'operates both as the inductance 11 and condenser 12; Moreovergin any'case, the capacity of the inductance-capacity combination must take into consideration. the

capacity of the coilemployed asthe-induq tance.' It is understood tance, and so this detector tube capacity is in i 7 parallel with condenser 12 and acts tot-in-r crease the effective value of this capacity. Accordingly, in any case, the capacity of'the inductance-capacity combination must takeinto consideration theca'paclty between the grid and filament of the detector'tube. Also, a

the inductance-capacity combination must take into account theeflect of the input cirr cuit that supplies the detector with the alterthat the grid to filament capacity of the detector tube is generally so situated in'relation to the inductance 11 as to be substantially in shunt with this inducnating current voltage or voltages to be detected. This is necessary becausethe charac terlstlcs of th1s input'circult may in some 011- cumstances affect the electrical characteristics;

of the grid circuit at the frequency of the converted current.

Now referring to Fig. 3,;a modified form of impedance for the grid is shown, 'in which the inductance is split into two portions 13 and 14, the entire inductance being shunted" by acondenser 15,and the portion 14: being shunted by a condenser 16. In this case, the total values of inductances and capacitiesare the same for the similar purposes as'thatdescribed in connection with'the inductance-- capacity combination of Fig.2; Fig. 4 discloses a further modification in Wl'llCll'lIl inductance '17 is used having in series therewith a condenser 18, this series being M shunted by a'condenser 19 and a grid leak 20; The combinations such as Figs. and 4 offer 1 advantage in certain cases in producing higher impedance totheiconverted current.

which there is a coil 21"shunted .by :a condenser 22 The coil or inductance 21 is coupled tojthe inductance 23', shunted by the condenser 24.- The coupled inductance adds its impedance into the main circuit with a In Fig. 5, a further modification of the impedance for the grid is illustratedyinf q multiplying factor determined by the amount of-coupling, and facilitates the productionof a high impedance to the convertedor rectified 1 current without substantial complica tions, and may be. employed where difiicul-f ties-are experienced in getting sufficient impedance'for proper detection. In the modifi- Y quencies, but will offer very low impedance to currents of all otherfrequencies', including:-

those which lie between theresonan't frequen-' :c1es.' I

"Now referring. to Fig. 6, a circuit is illustrated which is adapted to eliminate the tendencyofthe apparatus to produce oscilla- 'itions'. It will beobserved that with the de tector system ofthe present invention, it will frequently occur that both the grid andplate circuits will be in resonance with the rectified... current, and due tothe tube capacity; 7

there'will bepresent a tendency forthe detector. circuit-to produce oscillations, For

cationsshown in Figs. 3, 4:,and 5, it is, pos-Q siblei byusing' proper values of the induc- I tances and condensers, to formta c'ombination which is resonant at anumberfof fre quencies, and will therefore offer an extreme- 1y highimpedance to currents ofseveral fre-.

example, where the output of thedetector of f i Figs. 2 and '5, inclusive,is tobe amplified,-as in a superheterodyne circuit,,the plate cir-" .cuit of the-detectortube will be tuned to the frequency of the-converted current," and if p the converted current is sufficiently high in frequency, oscillations in the'detector maybe]. 'set Such 'vdifiiculties maybe aVO'idedb iIi,

several expedients. This may be accomplished either by inducing losses in the circuit to eliminate oscillation or by neutralizing the grid-to-plate capacity of the tube by any usual or preferred method. In Fig. 6, a detector tube 25 is indicated as having its grid 26 connected to an inductance-capacity combination similar to that illustrated in Fig. 2, and has its plate 27 connected with a transformer 28. The secondary of such transformer is connected through a capacity 29 back to the grid 26, the condenser 29 offering a voltage opposed to that of the feed back voltage in tube 25 for the purpose of eliminating undesirable oscillations.

In Fig. 7, neutralization is shown to be accomplished by using the so-called Rice method, 30 indicating a tube, 11 and 12 indicating the inductance condenser for the grid 31, and 32 indicates a condenser connected with the plate 33 of the tube and with the far side of the inductance 11.

In Fig. 8, oscillations are prevented by feeding back energy from the plate 34 to a coil 35 with the inductance 11 of the impedance combination 11, 12, said energy being opposed to that fed back to the capacity of the tube, and neutralization thus obtained.

It is understood that it is not always necessary or desirable to entirely neutralize the tendency of the detector system of the present invention to oscillate, inasmuch as a limited amount of regeneration is frequently useful. The detector therefore may be neutralized only sufiiciently to prevent oscillations, and frequently regeneration may be intentionally introduced in a variety of ways.

In order to illustrate one example of a complete apparatus embodying the present invention in Fig. 9, there is illustrated a circuit diagram of a radio receiver of the frequency changing or superheterodyne type, employing a detector principle of the present invention.

Referring to the figure, 41 indicates a tube having its grid 42 connected with an inductance 43 and condenser 44, which are of such sizes that, taking into account the grid to filament capacity of the detector tube and the distributed capacity of the coil, the path from the grid electrode to the filament electrode which must betraversed by the rectified current is tunedto offer a very high impedance to the frequency of this intermediate or rectified current, it being understood that condenser 44 is usually made of a capacity as small as possible while still remaining in excess of three or five times the filament-to-grid capacity of the tube 41. The detector tube 41 is indicated as at least partially neutralized by the Rice system, the plate 45 feeding back to condenser 46 to inductance 43 to oppose the regenerated voltage. 46 indicates a source of input, such as a transformer aerial and ground or loop to the circuit for imposing thereon a radio signal, and 47 indicates a form of oscillator circuit coupled with the grid circuit of tube 41 for applying a locally generated current to the detector tube 41 to aid in producing the intermediate or beat frequency for the receiver. It is understood that such system may be used for producing intermediate frequency either higher or lower than the frequency of the input signal, and that the detector may operate either by the so-called beat phenomena or may operate to produce a rectified current of the sum of the frequencies of the input frequency and the frequency of the oscillator 47. The detector tube 41 is indicated as connected in a conventional way to a radio frequency amplifier 48 of the converted or intermediate frequency, and said amplifier 48 is indicated as coupled in the conventional way to a detector 49 for detecting the audio frequency component of the signal being received. Said detector 49 may employ the detector sys tem of the present invention or the usual grid leak-condenser system, such system being effective for detecting audio frequencies. The detector is also indicated as connected as usual with an audio frequency amplifier 51.

Vfhile the forms of the invention herein described are well adapted for carrying out the objects of the present invention, it is understood that the invention is capable oi numerous modifications, and the invention therefore includes all such modifications and changes as come within the scope of the following appended claims.

I claim 1. A system for detecting oscillatory energy which comprises, a vacuum rectifying tube with input terminals, an input circuit connected therewith, and an output circuit for said rectifier, said input circuit including inductance condenser combination which is operated to establish resonance in the input circuit at the frequency of the rectified current in the output circuit, whereby to provide a high impedance in the input circuit to the rectified current flowing in said circuit, said inductance condenser combination offering a low impedance to the unrectified current in the input circuit.

2. An apparatus for detecting oscillatory energy which comprises, a vacuum tube, an input circuit for impressing the voltage for the energy to be detected across the filament and grid of said tube, an output circuit for said tube, said input circuit having an inductance condenser combination, the combination offering a low impedance to the frequency of the oscillations to be rectified and being adapted to establish resonance with the rectified current in the output circuit of the tube, and means for applying to the grit from the plate voltage counter to the regenerated voltage of the rectified current.

3. A frequency changing system comprising a detector, an input circuit forimposing oscillatory energyacross the filament andj v grid of said detector, means in said input circuit for imposing locally generated oscillatory energy, across the filament and grid V of said detector, an inductance capacity com- 4:. An apparatus of the class describe d comprising a vacuum tube having grid, filas bination in the input vcircuit adapted to bring" said circuit in resonance with; the'rectified current to be produced, said inductance capacity combination offering a low impedance to the input oscillatory energy and the locally c generated oscillatory energy.

ment anclplate electrodes, a circuit intercom necting the grid and filament electrodes in- I cluding means for passing a small direct current, means in said circuit for bringing said circuit in resonance with the rectified current to be produced, means n said c1rcu1t for'bypassing the frequency toYbe converted, and

means for imposing the rectified current from the plate circuit upon said first mentioned circuit, I r v 5. A system for detecting oscillatory energy which comprises, a vacuum rectifying tube with input termlnals and a c rcuit con-' nected between said input terminals, which circuit includes, means to'impre'ss the signal voltage on the input terminals of the tubes,

said circuit also including means, which, in COIlJllIlCt-IOD with the tubesrefiectlve input capacity, 1s operative to establlsh resonance in said circuit with a component of the rectified current, thereby to provide a high im-- pedance in the input circuit to the desired component of the rectified signal "current flowing in said circuit "connecting the input terminals. 1 V

Signed at Stanford University, Cal, this 17th day of February, 1927.

FREDERICK EMMONS TERMAN. 7 

